System and Method for Handoff Between Base Stations

ABSTRACT

A system and method for handoff are provided. A mobile station performs a make-before-break handoff of a control channel between a serving and target base station and a break-before-make handoff of a traffic channel between the serving and target base stations. The traffic channel handoff is performed after the control channel handoff has completed.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application is a divisional application of U.S. patentapplication Ser. No. 11/199,178, filed on Aug. 9, 2005, which claimspriority under 35 U.S.C. §119 to U.S. Provisional Application No.60/599,916, filed on Aug. 10, 2004, the entire disclosures of which areherein expressly incorporated by reference.

BACKGROUND AND SUMMARY OF THE INVENTION

In wireless communication systems, mobility between differentgeographical areas is provided by allowing mobile stations to handofftheir communications between base stations. The base station from whicha mobile station is handing off from is typically referred to as aserving base station, and the base station to which the mobile stationis to hand off to is typically referred to as a target base station.Mobile stations can include cellular telephones, personal digitalassistants (PDAs), computers equipped with wireless transceivers (suchas integrated transceivers or transceivers on PCMCIA cards), pagers, andthe like.

There are two general types of handoff, hard handoff and soft handoff.Hard handoff is performed in a break-before-make manner, in which amobile station ceases all traffic and control channel communicationswith the serving base station and then attempts to establish traffic andcontrol channel communications with the target base station. Thisapproach is used mostly to preserve air-link and network resourcesutilized by mobile stations in the handoff areas, and might be thepreferred mode for high-speed packet data channels. It also simplifiesthe handling of data packets at the network infrastructure. However,hard handoff does not provide full diversity gain associated with softhandoff, thereby increasing the possibility of temporary sessioninterruption. Such a temporary interruption would impact Quality ofService (QoS) for real-time applications, such as voice.

Using a soft handoff technique, a mobile station will simultaneouslycommunicate with the serving and target base stations over both basestations traffic and control channels. This type of handoff providesdiversity, as the mobile station can receive the same information fromboth base stations. Due to low chance of session interruption, softhandoff is typically preferred where real-time applications such asvoice are involved. However, soft handoff requires synchronous resourceallocation and scheduling, and coordinated packet handling on thenetwork side. This has a direct impact on throughput efficiency andnetwork complexity. Soft handoff also increases the required backhaulcapacity, especially for high rate traffic channels. Additionally, it isonly feasible when frequency reuse is 1:1 and strict trafficsynchronization is required among all bases stations involved in thesoft handoff. Moreover, soft handoff complicates packet scheduling andautomatic retransmission request (ARQ) messages.

While many wireless communication technologies provide either soft orhard handoff, broadband communication technologies including cdma2000,1xEV-DO, WCDMA, Flarion's Flash OFDM® and IEEE80216e provide both hardand soft handoff. Specifically, soft handoff is typically employed forcommunications that are sensitive to interruptions or errors, while hardhandoff is employed for all other communications.

Exemplary embodiments of the present invention provide a hybrid handoffscheme in which control channel communications are handed off from aserving base station to a target base station, while the mobile stationcontinues to perform data communications with the serving base station.Once the handoff of the control channel communications are completed,the data communications are handed off from the serving base station tothe target base station.

In accordance with one embodiment of the present invention, the servingbase station informs the mobile station of time periods, correspondingto frames, during which the mobile station can perform the signalingwith the target base station necessary for handing off the controlchannel communications from the serving base station to the target basestation. The serving base station then adjusts its scheduler in such away that the serving base station transmits unicast and multicastmessages to the mobile station during time periods other than thoseduring which the mobile station is performing signaling with the targetbase station.

In accordance with another embodiment of the present invention, a fastcell switch feedback channel is allocated to the mobile station whenthere is a possibility or a need for a handoff. The mobile station usesthe fast cell switch feedback channel to perform the signaling necessaryfor handing off the control channel communications from the serving basestation to the target base station. The mobile station then can handoffthe traffic channel communications from the serving base station to thetarget base station.

Other objects, advantages and novel features of the present inventionwill become apparent from the following detailed description of theinvention when considered in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a signaling diagram illustrating an exemplary method forhandoff of a mobile station between a serving base station and a targetbase station in accordance with one embodiment of the present invention;and

FIG. 2 is a signaling diagram illustrating an exemplary method forhandoff of a mobile station between a serving base station and a targetbase station in accordance with another embodiment of the presentinvention.

DETAILED DESCRIPTION OF THE DRAWINGS

FIG. 1 is a signaling diagram illustrating an exemplary method forhandoff of a mobile station between a serving base station and a targetbase station in accordance with one embodiment of the present invention.During the time period of Frame 1, the mobile station receives aneighbor advertisement (NBR-ADV) message broadcast by the serving basestation (BS1) identifying base stations for which the mobile station isto measure their signals. The mobile station can also receive thedownlink MAP (DL_MAP) and uplink MAP (UP_MAP) during Frame 1. The mobilestation then measures the received carrier-to-interference plus noiseratio (CINR) of all neighboring base stations advertised in the NBR-ADVmessage and transmits a mobile station handoff request (MSSHO-REQ)message during the time period of Frame 2. The MSSHO-REQ messageidentifies the measured base stations and their corresponding CINRs.

Base stations with CINR's above a predetermined threshold are consideredas “candidate” base stations for handoff. Based on the receivedmeasurements, and other logical considerations, the serving basestation, or the handoff controller entity in the network, selects one ofthe bases stations among the top candidate neighbor base stations as thetarget base station for handoff. The serving base station, which alsoserves as an “anchor” base station during the cell switching process,notifies the target base station (BS2) that the mobile station willattempt a handoff and transfer the mobile station's context to thetarget base station using, for example, HO Notification and ContextTransfer messages.

The target base station reserves a ranging channel and basic connectionidentifier (CID) for the mobile station, and forwards this informationto the serving base station. The serving base station, during the timeperiod of Frame 3, transmits a message to the mobile station instructingit to attempt a handoff to the target base station. The message includesthe target base station's identification, the reserved ranging channeland the basic CID. The reserved ranging channel can be a physicalchannel or a logical channel defined by a code. This message can be a HOPreparation Order Message, or can be an extension to a base stationhandoff response (BSHO-RSP) message. The message also identifiesupcoming time periods, or frames, in which the mobile station is allowedto be unreachable, herein referred to as “T-Frames”. The serving basestation's scheduler is adjusted to prevent scheduling any unicast ormulticast messages for transmission to the mobile station during theT-Frames. In FIG. 1 time periods, or frames, in which the mobile stationis to communicate with the serving base station are herein referred toas “S-Frames”. Although FIG. 1 illustrates the T-Frames occurringfollowing every other S-Frame, other schedules of the T-Frames arepossible.

Upon the occurrence of the first T-Frame, i.e., the time period of Frame4, the mobile station tunes to the control channel of the target basestation to receive broadcast control messages, such as the frame controlheader (FCH), DL_MAP and UL_MAP, and the like. The mobile station thensends an initial ranging message on the reserved ranging channel to thetarget base station. Although not illustrated, during the T-Frames, themobile station monitors broadcast information from the target basestation, such as the DL_MAP, UL_MAP, downlink channel descriptor (DCD),uplink channel descriptor (UCD), or any other signaling informationnecessary for handoff. Additionally, the serving base station, as partof the HO-Preparation message, can identify the current DCD and UCD ofthe target base station, if they are different from that of the servingbase station. Otherwise, the mobile station will have to monitor thetarget base station's downlink message until it receives the DCD andUCD. Knowing the DCD and UCD is necessary, as the mobile station cannotstart the initial ranging until it knows the DCD and UCD of the targetbase station.

The mobile station then tunes again to the serving base station duringthe next S-Frame, i.e., during the time period of Frame 5, in order toreceive any data from the serving base station. During the next T-Frame,i.e., the time period of Frame 6, the mobile station again tunes to thecontrol channel of the target base station to receive the response tothe ranging message (RNG-RSP), as well as a CID allocation. Although notillustrated, the mobile station and target base station can exchangeother Layer 2 and Layer 3 messages before the mobile station's trafficchannel is switched to the target base station. These messages may berelated to security, system parameter updates, or other informationneeded by the mobile station or the base station to carry the sessionseamlessly.

At the next S-Frame, i.e., the time period of Frame 7, the mobilestation tunes to the serving base station and transmits a messageindicating that the mobile station has completed the handoff of thecontrol channel to the target base station (HO-Completion Indication).The serving base station forwards an indication of the completed handoffto the target base station, at which point traffic information isswitched from the serving to target base station. The target basestation, during the time period of Frame 8, transmits a traffic channelallocation to the mobile station and the target base station becomes theserving base station for all traffic and control channel communicationsfor the mobile station.

When the method of FIG. 1 is performed between base stations thatoperate over the same frequency or frequencies, some message exchangesbetween the target base station and the mobile station can occur duringthe S-Frames. If the mobile station reselects the serving base stationbefore handoff to the target base station is completed, the serving basestation can terminate the handoff process by notifying the target basestation of the early termination. In this case, the serving base stationwill remain as the serving base station, traffic channel frame exchangeswill not be affected, and the mobile station will ignore the T-Frameallocation.

FIG. 2 is a signaling diagram illustrating an exemplary method forhandoff of a mobile station between a serving base station and a targetbase station in accordance with another embodiment of the presentinvention. In accordance with this embodiment of the present invention,a fast cell switch feedback channel is allocated to the mobile stationwhen there is a possibility or the need for handoff. This channel allowstimely and efficient notification of cell switching status, before,during and upon completion of the process.

As illustrated in FIG. 2, the mobile station receives NBR-ADV and DL_MAPand UL_MAP messages from the serving base station. The mobile stationthen measures the received CINR of all of neighboring base stationsadvertised in the broadcast NBR-ADV message. The mobile station thentransmits a MSSHO-REQ message identifying the measured base stations andtheir corresponding CINRs.

All reported neighbor base stations with CINR's above a predeterminedthreshold are considered as “candidate” base stations for handoff. Basedon the received measurements, and other logical considerations, the basestation specifies a subset of the candidate neighbors as recommendedbase stations for handoff, namely the Active Set. The serving basestation, which also serves as an “anchor” base station during the cellswitching process, allocates a dedicated channel named “Fast CellSwitch” or FCS to the mobile station. The Active Set and the FCS channelallocation can be specified through the BSHO-RSP message. The servingbase station also allocates a short, e.g. 3 bit, temporary ID for eachbase station in the Active Set. If the system defines an uplinkdedicated control channel for every active user, the FCS may also bedefined as a subchannel or field within uplink dedicated controlchannel.

The FCS channel is used by the mobile station to frequently indicate,e.g. every 5 msec or once a frame, one of the base stations in theActive Set with highest CINR to be the next target base station. Themobile station may consider other factors in selecting the target basestation from those in the Active Set.

Right after FCS allocation the mobile station may specify the currentanchor base station as the target base station for a period of timeuntil one of the Active Set base station's CINR become stronger than theserving base station. The system also defines a configurable timer forthe FCS to allow the network to perform necessary signaling with thetarget base station, and for the mobile station to establish dualsignaling with the serving base station and target base station in amake-before-break manner. The mobile station continues exchangingtraffic frames with the serving base station until that timer expires,or when it successfully completes the ranging with the target basestation.

If the mobile station reselects the serving base station again, beforehandoff to the target base station is competed, the serving base stationcan stop the cell switching process by notifying the target base stationof the early termination of handoff. Accordingly, the serving basestation does not change and traffic channel frame exchange will not beimpacted. During the handoff process the mobile station continuesmeasuring the signals from neighbor base stations and updates therecommended changes to, i.e., additions to and deletions from, thecurrent Active Set through MSSHO-REQ message.

New neighbor base stations will be added to Active Set if their CINR'sare above a predetermined threshold, and current Active Set members maybe deleted if their CINR's are below another predetermined threshold.The FCS will be automatically released by the mobile station once theserving base station becomes the only member of the Active Set.

When the mobile station notifies the serving base station of its desireto switch to the target base station, the serving base station transmitsa message to the target base station informing the target base stationof the handoff in process and to transfer the mobile station's contextinformation. The serving base station may also send a message (notillustrated) to the target base station identifying the FCS channellocation, so that the target base station can optionally listen to thischannel during the cell switching process.

Following the notification, the target base station allocates a basicCID and a reserved initial ranging CID for the mobile station, andtransmits this information to the serving base station. The serving basestation transmits this information to the mobile station. The mobilestation monitors the broadcast information from target base station,such as DL/UL_MAP, downlink channel descriptor (DCD), uplink channeldescriptor (UCD), or any other signaling information needed to switchthe traffic channel to target base station. To expedite the process ofswitching, the mobile station can start monitoring some of broadcastmessages from target base station as early as the time it sends theFCS=BS2 message to the serving base station. However, some ofinformation which requires a specific CID needs to be read once theearly target base station allocation, e.g. basic CID, are forwarded tomobile station from serving base station.

Moreover, the mobile station transmits a message to the target basestation on a reserved and contention-free ranging channel allowing thetarget base station to perform initial ranging. Once the mobile stationreceives the ranging response along with a CID allocation, it canindicate its handoff completion to the old serving base station bysending a predefined code word on the FCS channel. The mobile stationthen receives a traffic channel (TCH) allocation from the target basestation and the handoff process is completed.

As the base stations in the Active Set are given a temporary ID, e.g. 3bit TEMP_BS_ID, the mobile station can indicate its selected basestation on the FCS channel using this TEMP_BS_ID. One of TEMP_BS_ID's,e.g. 000, or a separate additional bit can be reserved to indicate thehandoff completion.

During the context transfer and ranging process with target basestation, the traffic communication is still performed between the mobilestation and serving base station, as long as the FCS timer is notexpired. After the interaction of necessary signaling information, andperforming initial ranging, and the allocation of necessary primary andtraffic CIDs for mobile station in target base station, the mobilestation indicates the handoff completion through the reserved ID on theFCS. The mobile station then releases that channel. This is the momentwhen the traffic communication is switched from the serving base stationto target base station.

Although the present invention has been described in connection with ahandoff between two different base stations, the present invention canalso be employed for handoffs between two sectors of the same basestation. It will be recognized that intra-base station communication andchannel establishments with the target sector can be performed fasterthan the inter-base station processes.

While the present invention is described in the context of an OFDMAsystem, and in IEEE 802.16 specifically, the present invention isequally applicable to other access technologies and standards.

The methods described above in connection with FIGS. 1 and 2 can be usedin, for example, high-speed packet data communications in thecontrol/signaling channels to preserve the link integrity throughout thehandoff process, while the hard handoff can be performed for the trafficchannels. However, if desired, for real-time and low-rate applications,such as voice, soft handoff can be performed by both traffic andsignaling channels.

The methods described above in connection with FIGS. 1 and 2 can be usedfor both handoff between two base stations with the same frequency,i.e., intra-frequency handoff, and between base stations of differentfrequencies, i.e., inter-frequency handoff. In both cases the describedmethod allows continuous active communications with the networkthroughout the handoff process, even for real-time and constant bit rateservices, possibly at lower data rates, but with negligible handofflatency.

The foregoing disclosure has been set forth merely to illustrate theinvention and is not intended to be limiting. Since modifications of thedisclosed embodiments incorporating the spirit and substance of theinvention may occur to persons skilled in the art, the invention shouldbe construed to include everything within the scope of the appendedclaims and equivalents thereof.

1. A method for a mobile station handoff from a serving base station toa target base station, the method comprising the acts of: performing, bythe mobile station, a make-before-break handoff of a control channelbetween the serving and target base stations; and performing, by themobile station, a break-before-make handoff of a traffic channel betweenthe serving and target base stations.
 2. The method of claim 1, whereinthe mobile station performs the handoff of traffic channels only afterthe mobile station performs the act of: indicating completion of thesignaling channel handoff.
 3. The method of claim 2, wherein thecompletion of the control channel handoff is performed over a handoffswitching channel.
 4. The method of claim 1, wherein the mobile station:receives data from the serving base station during a first and secondtime period; transmits control information to the target base stationduring a time period between the first and second time periods; andreceives data from the target base station during a time periodsubsequent to the second time period, wherein the mobile stationreceives data from only one base station during any particular timeperiod.
 5. The method of claim 4, further comprising the act of:receiving, from the serving base station, a message instructing themobile station to attempt to handoff to the target base station.
 6. Themethod of claim 5, wherein the message identifies a reserved rangingchannel of the target base station and a connection identification to beused for communicating with the target base station.
 7. The method ofclaim 6, wherein the message also identifies the time period between thefirst and second time periods.
 8. The method of claim 4, wherein themobile station receives a traffic channel allocation from the targetbase station during the time period subsequent to the second timeperiod.
 9. The method of claim 1, wherein the mobile station is informedof a first set of time periods for communicating with the target basestation over the control channel, information is scheduled fortransmission to the mobile station during time periods other than thefirst set of time periods, and the scheduled information is received bythe mobile station over a first traffic channel during periods of timeother than the first set of time periods.
 10. The method of claim 9,wherein the information for transmission to the mobile station includesunicast and multicast data.
 11. The method of claim 9, wherein the firstset of time periods are every other n frames.
 12. The method of claim 9,wherein the handoff is an intra-frequency handoff.
 13. The method ofclaim 9, wherein the handoff is an inter-frequency handoff.
 14. Themethod of claim 1, wherein the control channel is a first controlchannel and the traffic channel is a first traffic channel, and whereinthe mobile station: communicates with the serving base station over thefirst traffic channel and the first control channel during a first timeperiod; communicates with the target base station over a second controlchannel during a second time period; communicates with the serving basestation over the first traffic and control channels during a third timeperiod, which is subsequent to the second time period; and communicateswith the target base station over the second control channel and asecond traffic channel during a fourth time period.
 15. The method ofclaim 1, wherein the mobile station: identifies a base station as thetarget base station; exchanges control channel messages with the targetbase station; and communicates over the traffic channel with the servingbase station while exchanging the control channel messages with thetarget base station.
 16. The method of claim 15, further comprising theacts of: receiving a feedback channel allocation from the serving basestation; and transmitting an identification of the target base stationto the serving base station over the feedback channel.
 17. The method ofclaim 16, further comprising the act of: receiving, from the servingbase station, a signaling channel allocation for the target basestation, wherein at least one of the control channel messages is aranging message which is transmitted over the allocated signalingchannel.
 18. The method of claim 15, further comprising the acts of:receiving a connection identification from the target base station; andcommunicating over an allocated traffic channel with the target basestation.